A mirror can be made as thick as you like, a lens has limits
It's true that a large lens has flexure, but the rear surface of the lens almost perfectly cancels the aberrations from the front surface!
In ideal seeing conditions, the Yerkes and Lick refractors have both been regularly used at 1000x - 3000x, when examining extremely narrow double stars and both can split doubles down to their Dawes limit, which eminently show that the limit of lensmaking doesn't stop at 40", as is so often claimed.
But in physical size, a refractor larger than 40" is completely impractical, unless, of course, someone invents a design that can be made to much lower f/ number. Currently, that I know of, it's not possible to make special glass types normally used in apochromats in sizes much larger than ~12". The dialyte design shows promise, but still needs to be rather long, to keep other aberrations at bay. Peter Wise calculated a 60" design, that on paper should work well, but actually building it is of course another matter.